An apparatus of this kind is known from United Kingdom Pat. No. 1 435 144, for example, being used there as a heat-imaging apparatus. In this apparatus, the radiant or heat image is scanned bidirectionally; that is, it is scanned during both the forward and return movement of the scanning mirror. The scanning mirror is pivotable about two axes and is triggered in such a way that the heat image is scanned line by line, with successive lines being spaced apart from one another at right angles to the line direction. The known apparatus operates by the principle known as double-line interlacing, in which the scanning mirror is tilted by a small amount at the end point of one scanning movement, such as the forward pass, and then tilts back by the same amount at the end point of the follow-on scanning movement, in this instance the return pass.
This kind of scanning improves the resolution in the direction of the linear detector array, for example, with 120 detectors in a row, 240 scanning lines are obtained. Light-emitting elements are arranged in a row like the detectors and are used for generating the visible image.
Each individual detector of the linear detector array is associated with one light-emitting element via amplifier means. Because of the finite electrical band width of this amplifier chain, a phase displacement occurs between the signals generated by the detectors and those supplied to the light-emitting elements. As a result, the light-emitting elements light up in a delayed manner. In unidirectional scanning, this would not be a problem; however, in bidirectional scanning it has a very disruptive effect, because of the different scanning directions. For instance, with a scanning direction from left to right, a target in the form of a bar would be reproduced with a shift to the right, while in the next scanning line, it would be shifted to the left. The target, which in actuality is a straight line, would thus be reproduced as a wavy line.
For overcoming this disruptive effect, the scanning mirror in a heat-imaging device known from United Kingdom Pat. No. 1 435 144 is disposed in a housing that can be tilted about an axis which defines an angle of approximately 45.degree. with the line-scanning axis. The housing is fixedly connected to a lens that serves to project the light arriving from the light-emitting elements into an image plane. If the housing and hence the mirror are now tilted at the end point of a scanning movement, a movement component occurs that pivots the housing by a small amount in the line scanning direction. Thus, the lens connected with the housing pivots as well. All the components are matched to one another in such a way that this additional pivoting of the lens shifts the image in the image plane by precisely the amount that compensates for the phase error resulting from the phase displacement of the signal.
It is apparent that this kind of compensating apparatus is very difficult to adjust properly and that it requires very precise matching between the electrically produced and the optically produced effects. Furthermore, this apparatus can be used only for double-line interlacing, because compensation is possible only in two locations of the scanning lines, that is, the locations corresponding to the forward and return passes of the scanning mirror.
Another apparatus for compensating for the phase error caused by the phase displacement of the electrical signal is disclosed in German Pat. No. 2 805 531. In this apparatus, two rows of light-emitting elements are provided for generating the visible image, which are laterally displaced from one another in accordance with the error that is to be compensated and which are driven separately upon the forward and the return passes of the scanning mirror.
This apparatus has the disadvantage that the number of light-emitting elements relative to the number of detectors is doubled, so that the number of amplifiers and electrical connections must also be doubled, quite aside from any further electronic complexity for amplifier switching.
While this expense may still be tenable for an apparatus operating pursuant to the double-line interlacing principle, it becomes prohibitive if the resolution in the direction of the linear detector array is to be still further improved.